The semantics are basically the same as the methods defined for data frames in the
base package, with the only difference that the return values are data.set
objects. In fact, the methods described here are front-ends to the corresponding methods
for data frames, which are constructed such that the “extra” information attached to
variables within data.set objects, that is, to item objects.

data.set objects. On of the arguments to merge may also be an object coercable
into a data frame and the result still is a data.set object.

...

for subset: a logical vector of the same length as the number of rows of the
data.set and, optionally, a vector of variable names (tagged as select); for
unique: further arguments, ignored; for cbind, rbind: objects coercable into
data frames, with at least one being a data.set object; for merge: further
arguments such as arguments tagged with by, by.x, by.y, etc. that specify the
variables by which to merge the data sets of data frames x and y.

incomparables

a vector of values that cannot be compared. See unique.

deparse.level

an argument retained for reasons of compatibility of the default methods of cbind and
rbind.